Simplest DIY Li-Polymer Charger Schematic

[z-matrix]

hi guys,

it is not too safe what some are doing,
so i post here a simple cheap li charger for those who whould like to make one.

This one needs 6 components to work.

with very few modifications it can be made adjustable,
adjust charge current and cell voltage

get an LM317T
a 0.5% precision TL431 or similar (TS431)
and note that the CV sensing is a critical part of the circuit, so solder the connections well.

R4 and R5 connected to TL431 that is connected to LM317 adj, and ground prevents your li cell(s) from ignition.

parts list:

1x LM317
1x TL431 - 0.5%
1x 6k8 1% resistor
1x 10k 1% resistor
1x 1k resistor
1x current sensing resistor, for this 1.3A current, i used 2 x 0.47R 3w in series
use what you want

and you need a power sulpply with more than about 3V above output voltage

(optional 1u 100nF ceramic resistor)
(optional schottky diode before the output divisor)

[Norman Adlam]

Hi,

I think the simplest circuit I know of is using a L200 reg.
With a couple of resistors and the L200 you can set it for a current limited, voltage limited supply.

Cheers,

[Chippie]

I like the Scott Henion design based on an LM317 running in fixed voltage mode with a current reg using a transistor with sensing resistor.....

[AndyOne]

The L200 circuit has a built in current regulator so you only need one of them plus some simple passives. Here's some posts I made a while ago in the indoor and micro models forum on the L200 design.

http://www.rcgroups.com/forums/showpost.php?p=774657&postcount=23

http://www.rcgroups.com/forums/showpost.php?p=841401&postcount=121

http://www.rcgroups.com/forums/showpost.php?p=931641&postcount=180

http://www.rcgroups.com/forums/showpost.php?p=1709406&postcount=326

http://www.rcgroups.com/forums/showpost.php?p=1709409&postcount=327

Andy.

[z-matrix]

i prefer a 0.5% accurate voltage reference, that is thermally stable, and 1% divisor resistors.

a transistor base emitter junction has a break down voltage coefficient of -2.1mV/C (about 3.1% / 10C) and a fairly high diferential resistance with low beta.

this whould mean a sensing voltage decrease of 15mV at a 10C ambient temperature increase, or a +15mV shift at 10C lower temperature,
and a low output load regulation.

something like the set 4.2V will change between 4.06V to 4.33V if temperature is +-10C

[z-matrix]

Hi,

I think the simplest circuit I know of is using a L200 reg.
With a couple of resistors and the L200 you can set it for a current limited, voltage limited supply.

Cheers,

sorry i forgot to add parts list consisting of 6 components, though i thought it is very simple,

1x LM317
1x TL431 - 0.5%
1x 6k8 1% resistor
1x 10k 1% resistor
1x 1k resistor
1x current sensing resistor, for this 1.3A current, i used 2 x 0.47R 3w in series
use what you want

and you need a power sulpply with more than about 3V above output voltage

(optional 1u 100nF ceramic resistor)
(optional schottky diode before the output divisor)

so, this is a CC/CV circuit, this means in CC mode, it generates 1.3A, and when the voltage reaches CV voltage, it decreases current.

how does it work?
it sets a current limit with the lm317, and 1 resistor, and the voltage limit with the tl431, and 2 resistors.
the 1k resistor is needed to let the tl431 decrease the lm317 current, and to provide the neccesary current to the tl431.

[rc404]

I was looking for a simple charger for small 2S packs. I built this one last night and I definitely like it better than others I've seen. I'm not a EE, just know enough about electronics to be reasonably dangerous. So a couple questions.

Your last post mentions an optional Schottky diode. What is that for? Protect against reverse polarity hookup of the battery?

The second question relates to load regulation on the LM317. The data sheet I looked at gives specs at a minimum of 10 ma current. It also gives a generic circuit that uses a 240 ohm resistor and a 5K pot for adjustment. I'm wondering if the voltage divider that feeds the TL431 should be smaller resistors to provide at least some minimal (perhaps a few to 10 ma) load on the LM317? The thought would ensure good regulation even for a fully charged battery which was drawing little or no current.

regards

[z-matrix]

hi rc404

the optional schottky diode prevents current flowing back from battery to the charger circuit when power is disconnected, and reverse biasing of the LM317T.
Talking about the reverse polarity hookup:
this will discharge your pack, and you shouldn't do that, but a reverse polarity protection can be added, though i think it is not neccesary, noone will connect a li pack reverse.
And always check the noload voltage of your charger before charging unless you are sure what are you doing.

Reverse polarity detection can be done at the charge terminals, one hint i can give you is put an 1N4148 diode reverse biased in series with a little buzzer piezo and an optional current limiting resistor(if needed), so it will start tweeting if you plug in reverse.

Well, the first charger i built has an 1k and a 680R at the output divider, and it has a very good load regulation, output at no load was 4.21V maximum.

The LM317 is loaded by the TL431 while regulating if you look at the feedback loop.
The TL431 does the voltage regulation, LM317 stabilizes CC mode this is not critical part of regulation while charging a li-pack.

With one li cell the maximum cell voltage i measured was 4.2V, you have to leave it overnight to charge at this voltage level.

The circuit can be modified to be adjustable voltage / current by adding 2 potentiometers.
One in the current feedback loop, one in the voltage divider.
At the voltage divider, connect the center point of the 470R potentiometer to one end, and put in series with the 1k resistor if you use 1k, 680R in case 10k 6k8, use a 4k7 potentiometer.
this will let you set the cell voltage linearly using a linear (A) potentiometer,

current setting can be done either with a potentiometer by connecting the center tap in series with R3 resistor and connect the two endpoints to the leads of R2 now you can set current linearly using a linear (A) potentiometer, or a rotating switch with fixed resistors (this will give more efficiency at the resisitor side)

btw, i was testing a few things by adding a few operational amplifiers, and multiplying the voltage based on the connected balancing connector, it seemed all right plugged in a 2s , output voltage was 2 timesset cell voltage, plugging in 3s made the output voltage go 3 times set cell voltage.
Now i only have to add a balancing connector, and make a high efficinecy version using switched mode power conversion.

Is anyone intersted in a li cycler? ( charger-discharger )

[Chippie]

hi rc404

Is anyone intersted in a li cycler? ( charger-discharger )


Yup sure, bring it on:)

[rc404]

The LM317 is loaded by the TL431 while regulating if you look at the feedback loop.
The TL431 does the voltage regulation, LM317 stabilizes CC mode this is not critical part of regulation while charging a li-pack.



I see that now. Had not fully thought through how the circuit works. Its great!

Thanks

[Wisemo]

z-matrix or andy,
can your circuit be modified to handle a 10s battery? how so?

cheers
mo

[Dan Baldwin]

The TL431 has a maximum voltage of 36 volts, and in the circuit posted in the first post, the voltage on the TL431 would reach over 43 volts with a 10S lipo pack.

The maximum input voltage for the L200 is 40 volts, so Andy's charger wouldn't be appropriate for 10S either.

I have posted a couple of chargers, but they too are based on the TL431.

Dan